The Crosby Garrett helmet was sold to a private buyer because it was made of a copper alloy rather than gold or silver, thus not fitting the legal definition of treasure.
Photograph: Christie's/PA

When a breathtaking Roman helmet was discovered by an amateur metal detectorist in Cumbria in 2010, it sparked a desperate fundraising campaign to try to keep it on public display. The Crosby Garrett helmet was one of the most spectacular Roman artefacts ever found in Britain, but because it was made of a copper alloy rather than gold or silver, it did not fit the legal definition of treasure, which would have given a museum the first opportunity to buy it.

The government has now announced plans to widen the definition of treasure, so that more outstanding finds like the helmet can be preserved for the nation.

At present, only items that are at least 300 years old and made substantially of gold or silver, or which are found with artefacts of precious metals, can be declared treasure, so long as an owner cannot be found. Coins must be part of a hoard to qualify.

Under the new plans, that definition would be widened to include anything with a value over £10,000, meaning museums would be given first refusal on all valuable discoveries before they were offered in open sale to the highest bidder.

Announcing the plans, which will be open to public consultation until 30 April, the heritage minister, Michael Ellis, said: “These new proposals will help our museums acquire these treasures and make it harder for nationally important finds to be sold for profit.”

The Treasure Act 1996 obliges those who uncover finds that they know or suspect to be treasure to report them to the local coroner and gives museums an opportunity to buy them first at market rates, in a scheme administered by the British Museum.

Each year dozens of nationally important finds that do not fit the definition are thought to be lost to private buyers, with their finders under no obligation to report their discoveries.

Michael Lewis, head of the portable antiquities scheme at the British Museum, welcomed the proposed changes, saying: “Archaeologists have been keen for this to happen for some time, and from our perspective it’s a very positive thing.

“In the case of the Crosby Garrett helmet, for instance, the finder did tell us about it, but ultimately it’s not in a museum collection – it’s in private hands. If you or I wanted to go and see it, we can’t, and that’s a loss to our national heritage.”

Andrew Mackay, director of Tullie House, described the plans as “long overdue”. “It would have been fantastic to have allowed the public to access the helmet but ultimately it didn’t have the right gold or silver content. We welcome the news about the change in the act and hope this will ensure more national heritage can be secured for the public to enjoy.”